1. Field of the Invention
This invention relates to a noise suppressor using an active noise control under a noisy environmental condition.
2. Description of the Prior Art
Recently, the active noise control method has been proposed in which an environmental noise is subjected to noise suppression at the listening point thereof by outputting a control sound signal from a speaker using the digital signal technology.
A noise suppressor according to the prior art will be described below while referring to the drawings attached.
FIG. 4 is a block diagram of a noise suppressor of the prior art. In FIG. 4, 1a and 1b are microphones, 2 is an adaptive filter, 4 a speaker, and 8 a noise source. With the noise suppressor arranged as above, the operation thereof will be explained.
A noise signal detected by the microphone 1a is inputted to the adaptive filter 2 for adaptive control. The signal thus adaptively controlled is reproduced by the speaker 4. The sound signal thus reproduced is interfered with a noise signal outputted from the noise source 8 to generate an interference sound signal. The interference sound signal thus generated is detected by the microphone 1b. The signal thus detected is sent to the adaptive filter 2 as a control signal by an algorism such as, for example, the LMS method. The adaptive filter 2 performs the adaptive control so as to make the control signal small so that the noise signal at a position where the microphone 1b is located can be canceled with the sound signal reproduced by the speaker 4, resulting in a reduction in noise.
FIG. 5 is a block diagram of a noise suppressor according to the prior art, which is typically applied in a space where an audio signal is to be reproduced. In FIG. 5, 1a and 1b are microphones, 2 is an adaptive filter, 4a and 4b are speakers, 5 is audio equipment and 8 is a noise source.
This noise suppressor has the same operation as that shown in FIG. 4. In this case, however, a sound signal detected by the microphone 1b contains a noise signal from the noise source 8 and a canceling signal from the speaker 4 as well as additionally contains an audio signal from the speaker 4b. As a result, by being subjected to the influence of the audio signal which is independent of the noise signal to be inputted, a coefficient of the adaptive filter 2 cannot be convergently determined or can be varied, resulting in an unstable operation of adaptive control. As a result, in some cases, the noise may not be sufficiently attenuated or additionally increased on the contrary.
FIG. 6 is a block diagram of a noise suppressor which has an adder to be used in combination with a speaker additionally to that shown in FIG. 5. In FIG. 6, 1a and 1b are microphones, 2 an adaptive filter, 3 an adder, 4 a speaker, 5 an audio device and 8 a noise source.
Even in this case, the adaptive filter 2 cannot provide a stable adaptive control due to the influence of an audio signal from the audio device 5 as pointed out in FIG. 5, resulting in occurring such a case that the noise is insufficiently attenuated or additionally increased.
As explained above, with the noise suppressor according to the prior art, such a problem has been pointed out that when an audio signal is to be reproduced in a noisy environment, insufficient attenuation or unstable operation of adaptive control of a noise results.